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Chapters1 - Chemical Reactions and Equations
2 - Acids, Bases and Salts
3 - Metals and Non-metals
4 - Carbon and its Compounds
5 - Periodic Classification of Elements
6 - Life Processes
7 - Control and Coordination
8 - How do Organisms Reproduce
9 - Heredity and Evolution
10 - Light - Reflection and Refraction
11 - Human Eye and Colourful World
12 - Electricity
13 - Magnetic Effects of Electric Current
14 - Sources of Energy
15 - Our Environment
16 - Management of Natural Resources
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Chapter 12 - Electricity
What does an electric circuit mean?
Define the unit of current.
Calculate the number of electrons constituting one coulomb of charge.
Name a device that helps to maintain a potential difference across a conductor.
What is meant by saying that the potential difference between two points is 1 V?
How much energy is given to each coulomb of charge passing through a 6 V battery?
On what factors does the resistance of a conductor depend?
Will current flow more easily through a thick wire or a thin wire of the same material, when connected to the same source? Why?
Let the resistance of an electrical component remains constant while the potential difference across the two ends of the component decreases to half of its former value. What change will occur in the current through it?
Why are coils of electric toasters and electric irons made of an alloy rather than a pure metal?
Use the data in Table 12.2 to answer the following –
Table 12.2 Electrical resistivity of some substances at 20°C
|−||Material||Resistivity (Ω m)|
|Conductors||Silver||1.60 × 10−8|
|Copper||1.62 × 10−8|
|Aluminium||2.63 × 10−8|
|Tungsten||5.20 × 10−8|
|Nickel||6.84 × 10−8|
|Iron||10.0 × 10−8|
|Chromium||12.9 × 10−8|
|Mercury||94.0 × 10−8|
|Manganese||1.84 × 10−6|
(alloy of Cu and Ni)
|49 × 10−6|
(alloy of Cu, Mn and Ni)
|44 × 10−6|
(alloy of Ni, Cr, Mn and Fe)
|100 × 10−6|
|Glass||1010 − 1014|
|Insulators||Hard rubber||1013 − 1016|
|Ebonite||1015 − 1017|
|Diamond||1012 − 1013|
Draw a schematic diagram of a circuit consisting of a battery of three cells of 2 V each, a 5 Ω resistor, an 8 Ω resistor, and a 12 Ω resistor, and a plug key, all connected in series.
Redraw the circuit of question 1, putting in an ammeter to measure the current through the resistors and a voltmeter to measure potential difference across the 12 Ω resistor. What would be the readings in the ammeter and the voltmeter?
Judge the equivalent resistance when the following are connected in parallel − (a) 1 Ω and 106Ω, (b) 1 Ω and 103Ω and 106Ω.
An electric lamp of 100 Ω, a toaster of resistance 50 Ω, and a water filter of resistance 500 Ω are connected in parallel to a 220 Vsource. What is the resistance of an electric iron connected to the same source that takes as much current as all three appliances, and what is the current through it?
What are the advantages of connecting electrical devices in parallel with the battery instead of connecting them in series?
How can three resistors of resistances 2 Ω, 3 Ω and 6 Ω be connected to give a total resistance of (a) 4 Ω, (b) 1 Ω?
What is (a) the highest, (b) the lowest total resistance that can be secured by combinations of four coils of resistance 4 Ω, 8 Ω, 12 Ω, 24 Ω?
Why does the cord of an electric heater not glow while the heating element does?
Compute the heat generated while transferring 96000 coulomb of charge in one hour through a potential difference of 50 V.
An electric iron of resistance 20 Ω takes a current of 5 A. Calculate the heat developed in 30 s.
What determines the rate at which energy is delivered by a current?
An electric motor takes 5 A from a 220 V line. Determine the power of the motor and the energy consumed in 2 h.
Pages 221 - 222
A piece of wire of resistance R is cut into five equal parts. These parts are then connected in parallel. If the equivalent resistance of this combination is R’, then the ratio R/R’ is -
Which of the following terms does not represent electrical power in a circuit?
An electric bulb is rated 220 V and 100 W. When it is operated on 110 V, the power consumed will be –
(a) 100 W
(b) 75 W
(c) 50 W
(d) 25 W
Two conducting wires of the same material and of equal lengths and equal diameters are first connected in series and then parallel in a circuit across the same potential difference. The ratio of heat produced in series and parallel combinations would be –
How is a voltmeter connected in the circuit to measure the potential difference between two points?
A copper wire has diameter 0.5 mm and resistivity of 1.6 × 10−8Ω m. What will be the length of this wire to make its resistance 10 Ω? How much does the resistance change if the diameter is doubled?
The values of current I flowing in a given resistor for the corresponding values of
potential difference V across the resistor are given below –
Plot a graph between V and I and calculate the resistance of that resistor.
When a 12 V battery is connected across an unknown resistor, there is a current of 2.5 mA in the circuit. Find the value of the resistance of the resistor.
A battery of 9 V is connected in series with resistors of 0.2 Ω, 0.3 Ω, 0.4 Ω, 0.5 Ω and 12 Ω, respectively. How much current would flow through the 12 Ω resistor?
How many 176 Ω resistors (in parallel) are required to carry 5 A on a 220 V line?
Show how you would connect three resistors, each of resistance 6 Ω, so that the combination has a resistance of (i) 9 Ω, (ii) 4 Ω.
Several electric bulbs designed to be used on a 220 V electric supply line, are rated 10 W. Ω of 220 V line if the maximum allowable current is 5 A?
A hot plate of an electric oven connected to a 220 V line has two resistance coils A and B, each of 24 Ω resistances, which may be used separately, in series, or in parallel. What are the currents in the three cases?
Compare the power used in the 2 Ω resistor in each of the following circuits: (i) a 6 V battery in series with 1 Ω and 2 Ω resistors, and (ii) a 4 V battery in parallel with 12 Ω and 2 Ω resistors.
Which uses more energy, a 250 W TV set in 1 hr, or a 1200 W toaster in 10 minutes?
An electric heater of resistance 8 Ω draws 15 A from the service mains 2 hours. Calculate the rate at which heat is developed in the heater.
Explain Why is the tungsten used almost exclusively for filament of electric lamps?
Why are the conductors of electric heating devices, such as bread-toasters and electric irons, made of an alloy rather than a pure metal?
Why is the series arrangement not used for domestic circuits?
How does the resistance of a wire vary with its area of cross-section?
Why are copper and aluminium wires usually employed for electricity transmission?
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